Egypt's Muslim Brotherhood has claimed victory in the first round of the country's bitterly divisive constitutional referendum, with opposition forces complaining of large-scale rigging and violations.

Unofficial results from Saturday's first round showed 56% approval to 43% rejection on a low turnout of 33%, with a clear no win in Cairo, one of the 10 governorates where polling took place. The referendum is to be held in the country's remaining 17 governorates next Saturday - where prospects for a no win are poorer.

The figures were reported by the Freedom and Justice party (FJP), the political wing of the Brotherhood, which has been accurate in previous elections.

If, as expected, the trend is confirmed, the referendum will bolster the Brotherhood's Mohamed Morsi, who was elected president on a 51% mandate last June. But no end is in sight to the country's grave political crisis.

Sit-in protests against the constitution were continuing on Sunday night in Cairo's central Tahrir Square and at the presidential palace in Heliopolis, where nine people were killed last week.

The referendum was largely peaceful, but violence erupted on Saturday night when the Cairo headquarters of the liberal Wafd Party, part of the NSF, came under attack. The Wafd accused the Salafist preacher Hazem Abu Ismail of being involved but he denied responsibility.

If passed, the constitution will pave the way for new parliamentary elections next year. The last national assembly, dominated by the Brotherhood and Salafis, was dissolved. In a new election, many now expect a decline for the Islamist vote and a better performance by more confident and united opposition candidates. "The Brotherhood will not be able to run the country," predicted Kassem. "They will end up with state failure."

Egypt has been in turmoil, nearly two years after the fall of Hosni Mubarak, and Morsi's efforts to return the Arab world's biggest nation to normality have been hampered by what the opposition views as his autocratic tendencies.

The measure is generally expected to pass, given the well-organised Muslim Brotherhood's record of winning elections since the fall of Mubarak. But if the constitution is voted down, a new assembly will have to be formed to draft a revised version, a process that could take up to nine months.